1. Field of the Invention
This invention relates to solar cells and to a method of manufacturing the same.
2. Description of the Prior Art
The spectral energy of the solar radiation emitted from the sun generally has an energy intensity distribution as shown in a curve 100 of FIG. 1. In practice, the solar radiation energy distribution received at ground level has a distribution as shown by curve 102 in the same Figure, because a substantial amount of energy is partly absorbed by oxygen, carbon dioxide gas, or water vapour in the atmosphere, and partly scattered by molecules or atoms of the atmosphere. Curve 102 has a maximum value at a wavelength of about 500 nm. For silicon solar cells, the entire solar radiation energy shown in curve 102 is not utilized in practice for the generation of electric power. The silicon solar cell is sensitive to radiation of wavelengths between 350 and 1100 nm, and the spectral sensitivity curve and a maximum value thereof vary depending upon the silicon substrate, the crystal structure of the photoelectromotive force generating junction, the electronic structure and the manufacturing process.
At present, it is well known that research and development for matching the maximum in the spectral sensitivity distribution curve of the silicon solar cell to that of the maximum value of the spectral energy distribution of solar radiation energy is being actively carried out to increase the output of solar cells. On the other hand, it is also known in the art that an antireflective layer can be provided for increasing the output of a solar cell. Ideally, the reflectivity of the antireflective layer in a silicon solar cell, should be brought close to zero for the solar radiation of wavelengths between about 350 and 110 nm. Conventionally, a single antireflective layer of silicon monoxide, titanium dioxide or oxidized titanium, such as gas-reacted titanium monoxide or tanalum pentoxide, is used. But a single antireflective layer such as this cannot decrease the reflectivity sufficiently and therefore, the output of the solar cell cannot be increased satisfactorily.
For removing the above mentioned drawbacks, an antireflective layer having two different layers has been proposed in U.S. Pat. No. 4,055,422. FIG. 2 of this application shows such a solar cell, having an antireflective layer (104) comprising two layers (106 and 108). One layer (106) is made of a highly refractive material, such as titanium dioxide, having a refractive index of 2.35 to 2.40, and the other layer (108) is made of a low refractive index material, such as silicon oxide, having a refractive index of less than 1.7. The optical thickness nd (n=refractive index, d=real thickness) of layers 106 and 108 is equal to .lambda..sub.0 /4, wherein .lambda..sub.0 is equal to, for example, 600 nm. A glass plate (110) is mounted on the antireflective layer (104) by means of an adhesive material layer (112) as a protective means.
A spectral reflectivity distribution curve of antireflective layer 104 having a two layer structure is shown in FIG. 3. This curve is plotted with theoretical spectral reflectivity values in which the assumption is made that the reflected light permeates through adhesive layer 112.
As described hereinabove, the structures of conventional antireflective layers are proposed from the viewpoint of reducing the optical spectral reflectivity. Considerations made by the inventors show that the antireflective layer affects largely the carriers in the silicon substrate (114) of the cell shown in FIG. 2. The silicon substrate (114) and the antireflective layer (104) have in common an interface (116). A substrate of one conductivity type, for example a p-type silicon substrate (114), contains a principal surface (radiation receiving surface). The next or surface layer (118) is of opposite conductivity type, e.g. n+ type and therebetween a p-n junction is formed. On the n+ layer 118, a surface electrode (120) is provided to collect carriers generated by the solar radiation in the silicon substrate. A first antireflective layer (106) is formed to cover the n+ layer (118). This first antireflective layer (106) affects the lifetime of the carriers in the adjacent n+ layer (118) by means of an interface level through interface 116. This phenomenon is substantially the same phenomenon as that in which an injected carrier is affected by the surface conditions of an oxide film in a MOS varactor diode. In the case of the MOS diode, the oxide film is SiO.sub.2. When the recombination velocity of the carrier at the interface can be reduced and the lifetime of the carrier increased, the conversion efficiency of the solar cell is improved.